The present invention relates to crystal pulling apparatus for growing single crystal semiconductor material, and more particularly to a heat shield assembly for use in crystal puller apparatus for increasing the axial temperature gradient of single crystal semiconductor material grown in the apparatus.
Single crystal semiconductor material, which is the starting material for fabricating many electronic components, is commonly prepared using the Czochralski (xe2x80x9cCzxe2x80x9d) method. In this method, polycrystalline semiconductor source material such as polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is melted in a crucible. Then a seed crystal is lowered into the molten material (often referred to as the melt) and slowly raised to grow a single crystal ingot. As the ingot is grown, an upper end cone is formed by decreasing the pull rate and/or the melt temperature, thereby enlarging the ingot diameter, until a target diameter is reached. Once the target diameter is reached, the cylindrical main body of the ingot is formed by controlling the pull rate and the melt temperature to compensate for the decreasing melt level. Near the end of the growth process but before the crucible becomes empty, the ingot diameter is reduced to form a lower end cone which is separated from the melt to produce a finished ingot of semiconductor material.
To increase throughput of the crystal puller, it is desirable to increase the pull rate xe2x80x9cvxe2x80x9d at which the crystal is pulled up from the melt. However, simply increasing the pull rate, by itself, can be detrimental to the growth and quality of the crystal. For example, an increase in pull rate can result in distortion of the ingot diameter if the ingot is not given sufficient time to cool and solidify as it is pulled up from the melt.
Also, some wafer quality characteristics, such as Gate Oxide Integrity, are effected by a change in pull rate. Silicon wafers sliced from the ingot and manufactured according to conventional processes often include a silicon oxide layer formed on the surface of the wafer. Electronic circuit devices such as MOS devices are fabricated on this silicon oxide layer. Defects in the surface of the wafer, caused by agglomerations present in the growing crystal, lead to poor growth of the oxide layer. The quality of the oxide layer, often referred to as the oxide film dielectric breakdown strength, may be quantitatively measured by fabricating MOS devices on the oxide layer and testing the devices. The Gate Oxide Integrity (GOI) of the crystal is the percentage of operational devices on the oxide layer of the wafers processed from the crystal.
One way to improve GOI is to control the number of vacancies grown into the ingot upon solidification of the ingot as it is pulled up from the melt. It is understood that the type and initial concentration of vacancies and self-interstitials, which become fixed in the ingot as the ingot solidifies, are controlled by the ratio of the growth velocity (i.e., the pull rate v) to the local axial temperature gradient in the ingot at the time of solidification (Go). When the value of this ratio (v/Go) exceeds a critical value, the concentration of vacancies increases. Therefore, to inhibit an increase in the concentration of vacancies, i.e., to avoid increasing the ratio v/Go, the axial temperature gradient at the solid-liquid interface must be correspondingly increased if the pull rate v is to be increased.
To this end, it is known to provide a heat shield assembly disposed above the molten source material and surrounding the ingot as it is pulled upward from the source material to shield the ingot against heat radiated from the crucible and the heater surrounding the crucible. Heat shield assemblies are typically constructed of silicon carbide coated graphite. One disadvantage of these conventional heat shield assemblies is that graphite has a relatively high emissivity, i.e., it has a high ability to emit radiant energy from its surface. As a result, the graphite heat shield radiates a substantial amount of heat toward the ingot, thereby inhibiting cooling of the ingot as it is pulled up from the source material. Consequently, the pull rate at which ingots can be pulled up from the source material in crystal pullers incorporating a heat shield assembly constructed of graphite is limited.
Japanese Patent Application JP 8-325090-A discloses a single crystal drawing apparatus having a heat shield jig comprising a two-layer structure wherein an outer layer constructed of a first material, such as graphite, is coated with an inner layer of a second material, such as quartz, having an emissivity lower than the emissivity of the first material from which the outer layer is constructed. The application also discloses that instead of a quartz covered graphite construction, the two-layer construction could alternatively be constructed of a ceramic outer layer covered by molybdenum. Providing an inner layer of a material having a low emissivity results in decreased heat radiation from the heat shield jig toward the ingot, thereby allowing the ingot to cool more quickly. However, using such a multi-layered structure is inefficient in that the entire outer surface area of the inner layer of the heat shield jig is in thermally conductive contact with the entire inner surface area of the outer layer. Heat is therefore readily conducted from the outer layer to the inner layer of the heat shield jig, thereby reducing the effectiveness of the inner layer.
Among the several objects and features of the present invention may be noted the provision of a crystal puller which facilitates the growth of silicon crystals at elevated pull rates; and the provision of such a crystal puller which increases the axial temperature gradient of the crystal at the liquid-solid interface; the provision of a heat shield assembly for use in a crystal puller to increase the axial temperature gradient of the crystal at the liquid-solid interface; the provision of an inner reflector for a heat shield assembly of a crystal puller to increase the axial temperature gradient of the crystal at the liquid-solid interface; the provision of such an inner reflector that can be installed in and removed from the crystal puller independently from the remainder of the heat shield assembly; and the provision of such an inner reflector that can be used with existing heat shield assemblies of currently used crystal pullers.
In general, a reflector of the present invention for use in a heat shield assembly of a crystal puller for producing a monocrystalline ingot comprises a tubular structure adapted for positioning generally within the heat shield assembly and having a central opening sized and shaped for surrounding the ingot as the ingot is produced by the crystal puller. An outer surface of the tubular structure is adapted for placement in opposed relationship with the heat shield assembly. The tubular structure is constructed of a material having a low emissivity. The outer surface of the tubular structure has a spacer projecting outward therefrom for contacting the heat shield assembly to space the outer surface of the tubular structure from the heat shield assembly.
In another embodiment, a heat shield assembly of the present invention for use in a crystal puller for producing a monocrystalline ingot comprises an outer reflector having an inner surface and an outer surface. An inner reflector is adapted for positioning generally within the outer reflector in radially spaced relationship therewith. The inner reflector is constructed of a material having a low emissivity and has an outer surface in generally opposed relationship with the inner surface of the outer reflector. At least one of the outer surface of the inner reflector and the inner surface of the outer reflector has a spacer projecting outward therefrom and adapted for contact relationship between the inner reflector and the outer reflector. The spacer spaces the outer surface of the inner reflector from the inner surface of the outer reflector to inhibit heat conduction from the outer reflector to the inner reflector.
In yet another embodiment, a crystal puller of the present invention for producing a monocrystalline ingot comprises a crucible for holding molten semiconductor source material and a heater in thermal communication with the crucible for heating the crucible to a temperature sufficient to melt the semiconductor source material held by the crucible. A pulling mechanism is positioned above the crucible for pulling the ingot from the molten material held by the crucible. The heat shield assembly described above is adapted for location with respect to the crucible above the molten source material. The heat shield assembly has a central opening sized and shaped for surrounding the ingot as the ingot is pulled from the molten material and is generally interposed between the ingot and the crucible as the ingot is pulled from the source material.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.